Primary thyroid squamous cell carcinoma diagnosed with 18F-FDG PET/CT: a case report

Primary thyroid squamous cell carcinoma is extremely rare. We report a case of primary thyroid squamous cell carcinoma diagnosed using 18F-FDG PET/CT. The patient presented with left axillary lymphadenopathy as the initial symptom. Fine-needle aspiration of the axillary lymph nodes indicated metastatic squamous cell carcinoma. To identify the primary tumor, the patient underwent an 18F-FDG PET/CT scan, which revealed a mass in the thyroid and multiple enlarged lymph nodes with abnormal FDG uptake. Pathological examination of the axillary lymph nodes and thyroid biopsy confirmed the diagnosis of primary thyroid squamous cell carcinoma with lymph node metastasis.


Introduction
Primary thyroid squamous cell carcinoma is extremely rare, with an incidence rate of less than 1% (1), and it is a subtype of anaplastic thyroid carcinoma (2).It mostly occurs in elderly males (3), progresses rapidly, and has a very poor prognosis, with a survival period mostly shorter than one year (4,5).In this case, we describe a rare instance of primary thyroid squamous cell carcinoma with multiple systemic lymph node metastases.
To identify the primary tumor, the patient underwent 18 F-FDG PET/CT at our hospital.The PET/CT images showed a heterogeneous mass in the thyroid with significantly increased FDG uptake (Figures 2A-G, arrow), with an SUVmax of 10.0.Multiple enlarged lymph nodes in the neck, right supraclavicular fossa, left axilla, and abdomen,with an abnormally high FDG uptake, with an SUVmax of 15.3.No abnormal morphology or FDG metabolism was observed in other organs.Subsequently, the patient underwent fine-needle aspiration biopsy of the thyroid (Figures 1E, F), which revealed cancer cells with immunohistochemistry positive for P40.Based on the 18 F-FDG PET/CT and pathological findings, the patient was diagnosed with primary thyroid squamous cell carcinoma with multiple systemic lymph node metastases.The patient is currently undergoing the first cycle of chemotherapy with paclitaxel and carboplatin.

Discussion
Primary thyroid squamous cell carcinoma is a malignant tumor originating from thyroid tissue.In the latest World Health Organization classification of endocrine and neuroendocrine tumors, primary thyroid squamous cell carcinoma is included in anaplastic thyroid carcinoma,because squamous cell carcinoma of the thyroid often displays BRAF p.V600E mutations (87%) and exhibits positive immunohistochemical staining for the follicular cell markers PAX8 (91%) and TTF1 (38%) (6).Its incidence is extremely low, accounting for less than 1% of thyroid malignancies.Patients are mostly elderly males.However, it exhibits a high degree of malignancy, progresses rapidly, and carries a poor prognosis, with most patients surviving less than one year. 18F-FDG PET/CT can simultaneously display the glucose metabolism level and morphological information of lesions (7), which is valuable in the diagnosis and staging of thyroid cancer (8).There are few reports on 18 F-FDG PET/CT imaging of primary thyroid squamous cell carcinoma (9,10).
In this case, 18 F-FDG PET/CT was crucial in distinguishing primary thyroid squamous cell carcinoma from secondary squamous cell carcinoma.Secondary squamous cell carcinoma could originate from other squamous cell carcinomas outside the thyroid, such as those in the lungs, esophagus, or head and neck region.By showing increased FDG uptake in the thyroid gland and the absence of abnormal FDG uptake in other common sites of squamous cell carcinoma, 18 F-FDG PET/CT helped exclude the possibility of a metastatic origin.Additionally, the morphological information provided by PET/CT supported the localization of the tumor to the thyroid.Combined with pathological results showing squamous differentiation in the thyroid tissue, the diagnosis of primary thyroid squamous cell carcinoma was confirmed.

Conclusion
Primary thyroid squamous cell carcinoma is extremely rare,this case emphasizes the value of 18 F-FDG PET/CT in identifying primary tumor lesions and is also of great value in distinguishing between primary and secondary thyroid squamous cell carcinoma, which is of great significance for the diagnosis and staging of primary thyroid squamous cell carcinoma.

FIGURE 1 Images
FIGURE 1Images (A-D) show the pathological pictures of the left axillary lymph node biopsy.Images (E, F) show the pathological pictures of the thyroid biopsy.